This invention relates to a novel method of degassing a CRT (cathode-ray tube) with modified RF (radio frequency) heating of the mount assembly thereof during the simultaneous baking and exhausting of the CRT.
A CRT comprises an envelope which includes a neck, a funnel and a faceplate. A viewing screen and various coatings are applied to internal surfaces of the envelope. A mount assembly, supported from a glass stem and including an electron gun or guns, is sealed into the neck of the envelope. After the mount assembly is sealed into the neck, the CRT, which is open to the atmosphere through a glass tubulation connected to the stem, is baked at about 300.degree. to 450.degree. C. and is simultaneously exhausted to a relatively low pressure through the glass tubulation. During this baking, the temperature of the mount assembly rises to about 250.degree. to 300.degree. C. Then, the CRT is tipped off; that is, the tubulation is sealed. Near the end of the baking cycle and prior to tipping off, RF energy at about 1.0 to 1.5 megahertz is applied to degas metal structures, particularly the electrodes of the mount assembly. The RF energy heats the metal structures to temperatures above 450.degree. C., usually about 600.degree. to 750.degree. C., in order to drive out occluded and adsorbed gases.
A completed CRT, installed in a chassis, and operated in a normal manner, may continue to emit light from the viewing screen after the normal operating voltages are removed from the mount assembly. This effect, which may linger for minutes or hours, is referred to as afterglow and is attributed to the coincidence of two factors. First, a large residual electrostatic charge remains on the filter capacitor (which is integral with the CRT) after the operating voltages are removed, and therefore a residual high voltage remains on the anode of the CRT with respect to the other electrodes of the mount assembly. Second, there are sites on the electrodes of the electron gun from which electrons can be emitted when they are under the influence of the electric field produced by the residual charge on the filter capacitor. Emitted electrons under the influence of the electric field are directed toward and impinged upon the viewing screen producing the afterglow.